1. Field of the Invention
The present relates to an internal combustion engine with a dual turbocharger system having a first turbocharger and a second turbocharger wherein air is charged to the engine by only the first turbocharger at small intake air quantities and is charged by both the first and the second turbochargers at large intake air quantities.
2. Description of the Prior Art
Internal combustion engines with a dual turbocharger system are known by, for example, Japanese Patent Publications SHO 61-112734 and SHO 59-145328, and Japanese Utility Model Publication SHO 62-167329.
As illustrated in FIG. 13 (Prior Art), a conventional turbocharged internal combustion engine 1' is provided with a first turbocharger 7' and a second turbocharger 8' having turbines 7a' and 8a', respectively, and compressors 7b' and 8b', respectively. In an exhaust conduit 20b' downstream of the turbine 8a', and exhaust switching valve 17' is installed. Similarly, in an intake conduit 14b' downstream of the compressor 8b', an intake switching valve 18' is installed. Further, in an intake by-pass conduit 13' by-passing the compressor 8b', an intake by-pass valve 33' is installed. The intake switching valve 18' and the exhaust switching valve 17' are closed at small to medium intake air quantities to produce "one-turbocharger operation" wherein only the first turbocharger 7' is operated. The intake switching valve 18' and the exhaust switching valve 17' are opened at large intake air quantities to produce "two-turbocharger operation" wherein both the first turbocharger 7' and the second turbocharger 8' are operated. To obtain a smooth transition from the "one-turbocharger-operation" to "two-turbocharger-operation", the exhaust switching valve 17' is partially opened to run-up the second turbocharger 8.
However, there is the following problem in the above-described conventional turbocharged engine. In the case where the intake switching valve 18' is closed from the beginning of acceleration, the intake flow is introduced to the engine only through the first turbocharger 7'. Therefore, in acceleration from low engine loads when the first turbocharger 7' is not yet rotating at sufficiently high speeds, the compressor impeller of the first turbocharger 7' caused a resistance for the intake flow, thereby degrading the response characteristic at the beginning of the acceleration.